Janice Mc, I don't know why but the economy block has always been one of my favorites. I use them often but I do not do the complicated math calculations. I know I can, but why, when there is a handy, dandy little chart with all the calculations done already. I often use this site to help me with calculations, scroll down to the chart AKA cheat sheet The calculations have been perfect every time with just enough extra to square up to the 1/4in needed for the seam for the next round.
.https://catbirdquilts.wordpress.com/2014/01/26/economy-block-any-size-with-cheat-sheet/
For this block I cut the center square the incorrect size so had to expand it, then added side triangles and it was...blah. Spent some time with my friend "Jack" and added the little triangles and away I went. Squared up and ta-da! I just might try it again, purposefully and make the little focus fabric triangles larger. These were 2.5, I would likely go 3".
Give them a try they can become quite addictive.